Sunday, 3 August 2014

Pleistocene (Pt 16): Giant Wombats and Marsupial Lions

While Australia is not the only continent to have marsupials today - they're also found in the Americas - it certainly has the largest ones. This was, perhaps, even more true during the Ice Ages than it is today.

Of course, being an arid, tropical to subtropical, continent the Ice Ages affected Australia rather less than they affected Europe or North America, or even southern South America. There were no glaciers to be seen, and not a lot of snow unless you wanted to climb a mountain. On the other hand, there were some pretty big animals, including lizards and flightless birds larger than anything we have today. And, yes, the marsupials were bigger, too.

Many weren't that much larger than their modern equivalents - although, to be fair, that's quite large in the case of a kangaroo. But not all of them, for this was also the time of the largest marsupial ever to have lived: Diprotodon optatum, the giant wombat.
Strictly speaking, Diprotodon wasn't a wombat, in the sense that it wasn't a member of the wombat family we have today. Its distant ancestors were, in fact, probably about as closely related to koalas as they were to true wombats. This does at least put them in the "wombat-like" evolutionary branch of the herbivorous marsupials, rather than, say, the one with the kangaroos in it. So, given that they didn't climb gum trees like koalas did, wombats are about as close an analogy as you're going to find, and certainly among their closest living relatives.

Diprotodon was huge. We know from their skeletons that they were up to about twelve feet (3.7 metres) long from head to rump, and stood over well over six feet (2 metres) high at the shoulder. This is approximately the size of a hippo or a rhino, so that the only land-dwelling animal alive today that could dwarf them would be an elephant. Estimates of their exact weight are, of course, difficult to make when all you have is bones, but their solid, compact shape probably makes something like a hippo a fair analogy.

In 2001, a study estimated that a fully grown Diprotodon would have weighed a little over a metric ton, similar to a small rhino. Comparing this with other large animals revealed that this was rather lighter than might have been expected, something that the study's authors put down to the general lack of nutritious food in the Australian Outback. The assumption, in other words, was that they couldn't evolve to get any larger, because they would have starved.

However, a more recent study, shows that this is probably wrong, giving an estimate for the largest Diprotodon they could find of 2786 kg (3 US tons). Perhaps, they suggest, it had a relatively slow metabolism. Or perhaps we've underestimated how much food was available for them, since our current best guess is that they'd have needed to eat upwards of 100 kg (220 lbs) per day.

Diprotodon optatum was first discovered way back in 1836, by Richard Owen, more famous for coming up with the word "dinosaur" to describe some big fossil reptiles he'd found. Throughout the nineteenth century, multiple other species were named, all of them smaller, and often on the basis of subtle differences in the shape of the cheek teeth. More recently, it's become clear that many of these differences were too small to be worth counting, and the number of recognised species has dropped. Indeed, the description in 2009 of a single specimen with differently shaped cheek teeth on either side of its jaw goes to show that you probably want to be careful about this sort of thing.

Even so, that still left the issue of size. Some of the smaller Diprotodon were much older than the big ones, dating from the very beginning of the Ice Age. It's unclear whether they are really different enough to represent a separate species, but it seems likely that they represent at least a transitional form, half way between evolving from an immediate ancestor like Euryzygoma to something much larger. In other words, Diprotodon may have got bigger as time went on, reaching its maximum size just before it went extinct.

However, some smaller animals were of the same age as the really large specimens, and, for a long time it looked like there were two different species at least. We now think otherwise: it seems that the smaller ones were females. It's the sort of misunderstanding that can arise when all you have to go on is fossils. This difference in size between the sexes may suggest that Diprotodon travelled in single-sex herds, with the males competing for a harem of females during the breeding season, needing the extra bulk and muscle to drive away rivals.

So there's only one species. But what was it like, aside from being a hippo-sized wombat? Remains have been found right across the continent, which suggests that it was a fairly adaptable animal, although it likely preferred relatively open terrain to dense forest or jungle. It was a herbivore, and had huge chisel-like front teeth, similar to those on living rodents (as do most herbivorous marsupials, come to that, although they're pretty noticeable on something this size). Presumably, given their adaptability, they ate pretty much any plant material they came across.

Being so large, they can't have had much to fear from predators, either. Today, the largest predator in Australia is the dingo, which has lived on the continent for no more than a few thousand years, apparently arriving quite a lot later than the first humans. Next down, and the largest marsupial carnivore today, is the Tasmanian devil, which is no bigger than a small dog. While they're certainly a danger to modern wombats, its unlikely that one the size of a hippo would have had much to fear.

Back in the Pleistocene, however, there were marsupial carnivores somewhat larger than a Tasmanian devil. Enter the marsupial lion, Thylacoleo carnifex, perhaps the most deadly marsupial predator ever.

Despite the name, they were not particularly lion-like. Indeed, they arguably aren't even the most lion-like marsupials to have lived. But, to be honest, there isn't much around today that they were like. Indeed, their evolutionary history is a bit odd, because they appear to be more closely related to the "marsupial herbivores", distant relatives of kangaroos, possums, and, indeed, wombats, than they are to Tasmanian devils and their ilk.

Like Diprotodon, this group of animals is known for its chisel-like front teeth. In the marsupial lion, however, they have become adapted, not to clipping tough vegetation, but to biting into solid flesh. In effect, they are acting as canine teeth, since the real canines are unusually small. Presumably this is because their herbivorous ancestors didn't need large canines, and many living marsupial herbivores don't have canine teeth at all.

Behind these canine-like incisors, however, are the cheek teeth, which are even more highly modified. In marsupial lions, they formed elongated cutting blades, much like the 'true' carnassial teeth of placental carnivores, but larger, longer, and, if anything, even more deadly. They were great shearing, meat-chopping cleavers that could, size for size, deliver the most powerful bite of any known mammal, alive or dead. Thylacoleo was therefore, by at least some standards, the most specialised mammalian predator there has ever been.

Given that, it's a fair bet that marsupial lions didn't go after small prey. They were, very roughly, about the size of a leopard, but much more heavily built, perhaps weighing somewhere between 100 and 130 kg (220 to 290 lbs) - about the same as a lioness. The limbs were similarly proportioned to those of lions, with a long muscular tail for counterbalance, suggesting that they could chase prey for short distances, or perhaps pounce on them from cover. So, while their stocky bodies, powerful tails, sloping foreheads and long incisors wouldn't have made them look very lion-like, it does seem that they at least lived a similar lifestyle.

Once they had hold of a victim, they likely killed it in much the same way that sabretooths did, slashing the throat open with their front teeth, and then devouring the carcass. Their overall anatomy suggests that they would have been more than capable of taking down animals larger than themselves, with their only dietary limitation likely being that they couldn't eat anything that was too small. Just how big could their victims be? Well, as big as anything they could actually have met. Granted, it's possible that the tooth marks that they left on Diprotodon bones could have been the result of eating carrion after the giant wombat had died... but that's by no means certain.

Both giant wombats and marsupial lions, therefore, were adaptable and successful animals that could eat largely whatever they wanted. Which makes one wonder why they went extinct. The same could be asked of many North American animals, and there, the great wave of extinction occurred around 12,000 years ago, as the Ice Ages ended and humans arrived. That likely provided a double-whammy, with animals suddenly being hunted just as they faced a dramatically changing climate. In Australia it happened earlier, with Diprotodon, and many others, apparently going extinct around 44,000 BC. This matches up with the arrival of the first aborigines, but, significantly, it's about 30,000 years too early for climate change to have played a major part. On this continent, it seems, it's harder to get humans even partially off the hook.

Much of this hinges, though, on whether 44,000 BC really is the right date, and it's by no means settled that it is. There is some evidence that large Australian animals did survive for much longer than this. Some fossils seem to date from as recently as 20,000 years ago, and there are what may be aboriginal cave paintings of animals like Diprotodon. It's even been suggested that giant wombats might be the origin of the bunyip myth, although there are certainly other options.

Thylacoleo, too, seems to have survived long enough to be depicted in contemporary art. Assuming it really is what we think it is - and it does look more like a marsupial lion than a thylacine, which is the only other candidate - then this adds a piece of information we would never have known from the fossils: apparently, marsupial lions were striped.

However, many of these late fossils and human depictions come from the margins of the continent, places such as Kangaroo Island off the south coast. This would suggest that the animals were already being driven to extinction - whether by direct hunting, or, perhaps more likely, by simple competition with humans - well before the climate began to change.

The debate continues as to which had the bigger impact. But, while the situation in North America does look like a mix of the two, and climate change looks like the bigger culprit in Europe, for my money, in Australia it does kind of look like it was us. Not for the last time.

With this, the sixteenth part in the series, I have said basically all I need to for the time being about the Pleistocene, the time of the great Ice Ages. There are three continents that I didn't really cover, except in passing. Pleistocene Asia was, on the whole, not that different from Europe. The animals there weren't identical, by any means, but they were certainly similar. The story of Ice Age South America is essentially one of all the really cool animals dying off right at the beginning, or else moving north, where I described them anyway, and then the modern jaguars and llamas and so on moving in to replace them.

And Ice Age Antarctica... had penguins.

The Pleistocene ends, by current agreement, around 9,700 BC. By this time, humans were on every continent, save Antarctica, and the rapidly improving climate seems to have led to the development of agriculture in a number of different places across the globe at around the same time. From here on in, we're geologically in the Holocene, and culturally in the Neolithic. About half way through, we'll reach the Bronze Age and the dawn of civilisation, and everything else is history. Literally.[Picture by Dimitri Bogdanov, from Wikimedia Commons]